1. Field of the Invention
The present invention relates to compositions and methods for treating water, in particular recirculated water, such as that found in pools, hot tubs, spas, and process equipment, such as cooling towers, to control the growth of algae by contacting the water with compositions containing lanthanide halide salts.
2. Description of the Related Art
Phosphorus compounds, and in particular phosphates, are nutrients required for all living protoplasm, which contains about 2% phosphorus on a dry basis. Insufficient phosphorus can thus limit the growth and development of aquatic organisms. Phosphate compounds find their way into water via various methods. For example, phosphate is used as a nutrient in biological waste treatment processes, as industrial water softening chemicals, as complexing agents in boiler waters, as threshold treatment agents, and as builders in detergent compositions. They can also be introduced into water through animal waste products, such as urine and sweat, as well as through the breakdown of plant material or water treatment chemicals added to the water.
The presence of significant levels of phosphate compounds in water can lead to eutrophication--the phosphates provide nutrients for the development and growth of undesirable algae and other microorganisms. In streams, lakes, and ponds, this can result in oxygen depletion and destruction of valuable aquatic plant and animal life. In recreational bodies of water, such as in pools, hot tubs, and spas, the growth of algae is unsightly and can clog filters and pumps, create health concerns, and effectively make the pool, hot tub, or spa unusable. In process equipment using recirculated water, such as in cooling towers, the uncontrolled growth of algae and other microorganisms can decrease efficiency of heat transfer surfaces, increase load on pumps and other equipment, and create health concerns about the transfer of the microorganisms between heat transfer fluids, e.g., from the cooling water to the cooled air.
One way to control the growth of algae and other microorganisms in water is to control the amount of nutrients present. For instance, control of the level of phosphate compounds by effectively removing excess phosphates from the water can have a significant effect on algae and microorganism control. In some cases, control of the phosphorus level in the water may be the only effective way to control the development of algae and other microorganisms, short of adding biocides. For example, there is some evidence the blue-green algae are capable of satisfying their demand for nitrogen-containing nutrients by removing atmospheric nitrogen from the air. Thus, control of nitrogen nutrients in the water is unlikely to provide adequate control of these organisms, leading to the need for control of phosphate compounds instead.
Algal growth leads first to a coating of slime on the surfaces containing the water, such as on the walls and bottom of a swimming pool, hot tub, or spa, then to an unpleasant green discoloration of the water, and finally to the proliferation of other microorganisms and aquatic species, some of which may be pathogenic for humans. Methods for controlling algae include physically scrubbing the surfaces where algae grow and then pumping the water through a filter to remove the dislodged algae from the water. Alternatively, chemical toxins or biocides are added to the water to kill the algae and other microorganisms. In many cases, pool, hot tub, or spa owners use some combination of these treatments. However, the use of toxins or biocides typically causes some concern with respect to how these materials will affect the health of those using the pool, tub, or spa.
The difficulty of maintaining suitably clean, algae-free water in a pool, hot tub, or spa is generally considered to increase with the age and usage of the pool, tub, or spa The water of heavily used pools tends to develop an unpleasant odor and to irritate the eyes and skin of users, due at least in part to the development of high levels of chloramines in the pools, a result of the use of large quantities of chlorine-containing compounds as biocides. Extended "super-chlorination" can be used to remedy this problem, but this renders the pool unusable for a substantial time, and is only a temporary solution, since the chloramine and algae soon return.
Accordingly, there is a need in the area of treating water, in particular pool, hot tub, and spa water, as well as process water, such as water in cooling towers, for a composition and technique that controls the formation of algae without the need for mechanical scrubbing or for toxic biocides. In particular, there is a need for compositions and methods that control the level of phosphates in the water, thereby limiting the nutrient available for the growth of algae and other microorganisms. Phosphate levels in this type of water can range from a few parts per billion (ppb) to over 1000 ppb. Algae development typically becomes supportable at phosphate levels of about 20 ppb, and becomes very objectionable at levels above 400 ppb.
One approach to control of the phosphate concentration in water has been to add to the water compounds that are capable of precipitating all or most of the phosphate as an insoluble salt. These systems are somewhat complex due to the nature of the various phosphate compounds that may be present, such as orthophosphate (present as H.sub.3 PO.sub.4, H.sub.2 PO.sub.4.sup.-, HPO.sub.4.sup.2-, PO.sub.4.sup.3-, HPO.sub.4.sup.2-, and complexes thereof), pyrophosphate (present as H.sub.4 P.sub.2 O.sub.7, H.sub.3 P.sub.2 O.sub.7.sup.-, H.sub.2 P.sub.2 O.sub.7.sup.2-, HP.sub.2 O.sub.7.sup.3-, P.sub.2 O.sub.7.sup.4-, and complexes thereof), tripolyphosphate (present as H.sub.3 P.sub.3 O.sub.10.sup.2-, H.sub.2 P.sub.3 O.sub.10.sup.3-, HP.sub.3 O.sub.10.sup.4-, P.sub.3 O.sub.10.sup.5-, and complexes thereof), trimetaphosphate (present as HP.sub.3 O.sub.9.sup.2- and P.sub.3 O.sub.9.sup.3-) and organic phosphates (such as phospholipids, sugar phosphates, nucleotides, phosphoamides, etc.). Additional complexity is introduced by the acid-base equilibria of phosphoric acid. This complexity makes difficult prediction of the results obtained by treating phosphate-containing water with salts to form insoluble precipitates.
For example, attempts to use calcium salts, such as lime, to precipitate phosphate as hydroxyapatite are not as successful as an analysis of solubility product constants might lead one to believe, at least in part due to nucleation and crystallinity considerations. Moreover, the use of calcium, iron, and aluminum salts to precipitate phosphate is typically restricted to a fairly narrow pH range (in the acidic range, below the recommended pH for pool water for iron and aluminum, and in the basic range above the recommended pH for calcium), requires the addition of large quantities of salts, and produces large quantities of flocs that are unsightly and tend to clog filters. These techniques are not particularly effective at reducing the phosphate level below 50 ppb, leaving sufficient phosphate in the water to support continued algal growth, and are difficult to use in pool water due to the buffering compounds typically present in pool water to maintain at the recommended pH.
Lanthanum nitrate has recently been suggested for the removal of phosphate compounds from pure aqueous phosphate solutions and from activated sludge-treated wastewater by precipitation. However, this treatment would not be particularly suitable for use in swimming pools, hot tubs, or spas, since the reaction between the lanthanum salt and polyphosphates can fail to form settlable flocs outside of the optimum pH region, requiring the pool owner to either precisely control the pH of the pool water, or to live with turbidity from the suspended lanthanum phosphate salts. In addition, the pH for maximum phosphate removal using a 1:1 lanthanum: phosphate ratio is well below the pH of most pools, and a substantial amount of residual turbidity is observed in the pH range of most pool water. Thus, this technique is not suitable for a constant, steady treatment of the water to remove phosphate over time.
Lanthanum carbonate has also recently been suggested to treat pool water to reduce algae and orthophosphate levels in combination with aluminum chloride. However, this salt also causes a substantial amount of pool turbidity, which is undesirable. The aluminum chloride also contributes substantially to the level of removal achieved. In addition, the efficiency with which lanthanum carbonate removes phosphate appears to be enhanced by the presence of high concentrations of chloride ion, and appears to decrease when lanthanum carbonate is used in fresh water pools or other bodies of fresh water. It has been suggested that lanthanum carbonate removes orthophosphate from water via an ion exchange mechanism. However, an ion exchange mechanism is not suitable for removal of polyphosphates, which can form a significant quantity of phosphate in the water.
Accordingly, there remains a need in the art for a water treatment composition and method for reducing algal growth and removing phosphate from the water consistently over time, without the need for mechanical, biocidal chemical, or super-chlorination treatment, and which does not leave the pool water turbid.